A non-linear diffusion equation for interstitial atoms in metals was derived in which higher spatial derivatives with respect to concentration were taken into account. These were shown to make a significant contribution to the formation of modulated structures during phase separation.

Equation of Diffusion of Interstitial Atoms in Metals at Arbitrary Concentration Gradients. L.I.Smirnov: Physics of Metals and Metallography, 1998, 86[5], 446-8